This invention relates to an ultrathin-wall fluoropolymer tube having a removable fluoropolymer core and multilayered tubes having such a fluoropolymer tube as an inner lubricious layer.
Polytetrafluoroethylene and related fluoropolymers possess a unique combination of properties that make this material desirable for a variety of applications. It is inert to almost all chemical solvents, is mechanically strong and tough, has exceptional dielectric properties, can be used at high temperatures, and has an exceptionally low coefficient of friction. Such a combination of properties has thus made this material particularly suited for application as wire coatings and tubing, and especially as an inner liner or sheath in multilayered tubes such as automotive cables and catheters where it is desired to have an inner lubricious surface through which a cable or medical instrument can be freely guided. When applied to medical catheters, it is particularly important to be able to form a fluoropolymer tube with as thin a wall as possible so that the catheter can have a maximum inner diameter and a minimum overall outer diameter.
Typically, fluoropolymer tubes are manufactured utilizing a free extrusion. That is, the fluoropolymer is extruded in the form of a hollow tube, then subjected to further processing. This process is satisfactory for tubes with relatively thick walls, but impractical for use in manufacturing thin wall fluoropolymer tubes with wall thicknesses less than 0.002 inch (0.051 mm). As wall thickness decreases, processing by free extrusion becomes much more difficult since the tube can kink or collapse or flatten during further processing and handling. Also, with free extrusion it is difficult to control the interior diameter of the tube to tight tolerances, for example .+-.0.001 inch (0.0025 mm) or less, which is required for many applications, particularly medical catheters.
Another technique which has been employed to manufacture polymeric tubes, particularly where interior diameter control is important, is to coat the polymer over a metal wire core (or mandrel), either by extrusion of the polymer over the wire or by dipping the wire into a polymer bath, then removing the wire core from the cured polymer by stretching the wire to reduce its diameter and sliding off the polymer tube. The use of a wire core in this technique not only provides better interior diameter control, but also provides a support for the tube during subsequent processing and handling and prevents the tube from collapsing or kinking.
Unfortunately, this technique has a number of disadvantages. Stretching and removing the wire core is a cumbersome process and is impractical to utilize with long lengths of tube. It may be difficult or impossible to remove the core from other than relatively short straight lengths of tube. Obviously, if an entire spool of tube must be cut to shorter lengths and the core removed from each cut piece, this will be an inefficient, time consuming process and will waste product, which in turn will increase the cost of the final product. Also, before the metal core can be removed from the tube, the cut end of the metal core must be carefully dressed so that no barb is present that would tear or damage the interior surface of the tube as it passes over the cut end. This dressing further diminishes the efficiency of the process. In addition to the above difficulties, the wire core will also leave trace amounts of metal or other contaminants on the interior surface of the tube after it is removed. This contamination will have to be removed, such as by flushing with acid, if the tube is to be used for sensitive applications such as a medical catheter. This additional step also greatly diminishes the efficiency of the process and increases the cost of the final product.
It would be highly desirable if a fluoropolymer tube, particularly an ultrathin-wall fluoropolymer tube, could be formed over an easily removable core material. A fluoropolymer core would generally not be expected to be suitable for this purpose since fluoropolymers normally bond together when subjected to sintering temperatures. Thus, there is currently no known core material that will serve as an easily removable mandrel for fluoropolymer tubes without the disadvantages associated with known metal mandrels.